ERA24LA205 · Airbus Helicopters Inc AS350B3 B3e

During the enroute portion of the flight, the pilot in the left seat of the helicopter requested a transfer of control from the pilot in the right seat so that he could gain proficiency in preparation for obtaining a flight instructor certificate. After a positive transfer of control, the pilot in the left seat successfully conducted a confined-area approach to a 15-ft hover, then departed to search for an area to conduct a second approach. When the helicopter was about 50 ft above ground level (agl) and at an airspeed of about 20 kts, its forward airspeed slowed, and it entered a left turn that the pilot’s control inputs were ineffective in arresting. The turn progressed into a full rotation to the left and the helicopter completed two or three full rotations before impacting the ground, separating the right skid and tail rotor and damaging the tailboom and lower vertical fin. Postaccident examination of the airframe revealed that all damage features observed on the tailboom, aft ring frame, and tail rotor gearbox were consistent with overload structural failures sustained during the hard landing. The separated tail rotor blade did not show significant fragmentation, consistent with unpowered separation of the blade, likely due to impact with the ground after the tail rotor gearbox separated from the airframe. The tail rotor control system was continuous to the point where the tail rotor gearbox had separated from the airframe. One of six attachment bolts for the No. 1 tail rotor driveshaft (TRDS) hanger bearing support was not present in its installed location. The bolt was found lying in the tray area of the TRDS cover, and the respective nut was not found. The five remaining attachment bolts for the No. 1 TRDS hanger bearing support were found loose, consistent with having been inadequately torqued when installed during maintenance activities performed about 1 month before the accident. However, the tail rotor driveshafts showed no evidence of anomalous wear or preimpact fracture; thus, it is unlikely that the improper maintenance of the No. 1 TRDS hanger bearing support attachment hardware was a factor in this accident. No evidence of a pre-accident mechanical malfunction of the helicopter, control systems, or components that would have precluded normal operation was identified. Although the pilot stated that his control inputs were ineffective to counter the helicopter’s uncommanded left turn, data recovered from the engine data recorder (EDR) showed that the tail rotor potentiometer (XPA) value peaked only at 58.82% (consistent with a pedal position value of only 74.5%) and for only 1 second. This indicated that the pilot did not push the right pedal to its fullest extent and did not push it long enough for the pedal input to effectively counter the uncommanded left turn. According to a safety notice published by the helicopter manufacturer, all types of single-rotor helicopters can be susceptible to unanticipated left yaw at low speed. The notice indicated that unanticipated yaw has historically been referred to as “loss of tail rotor effectiveness,” but noted that the term “wrongly implies that tail rotor efficiency is reduced.” The notice stated that, when an unanticipated left yaw situation is encountered, “swift corrective action is needed in response, otherwise loss of control and possible accident may result.” According to the notice, “the key feature of an unanticipated left yaw recovery is large amplitude right pedal input. Recovery may not be immediate but will occur if the pilot persists in maintaining right pedal.” The helicopter was equipped with a cockpit image recorder that could have provided additional data to better understand the sequence of events leading up to the accident. However, it did not record the accident flight or any other flights in the previous 90 days and would not record when examined after the accident. The required annual functional check of the system, which maintenance records indicated was performed about 1 month before the accident, should have detected and rectified the malfunction before returning the helicopter to service.

Verbatim NTSB analysis and probable cause from the NTSB dataset